commit 3ff3b9997cfef891ba33a14f1dcba0310d96369c
Author: Joseph Myers <josmyers@redhat.com>
Date:   Tue Jan 28 23:20:08 2025 +0000

    Fix fflush handling for mmap files after ungetc (bug 32535)
    
    As discussed in bug 32535, fflush fails on files opened for reading
    using mmap after ungetc.  Fix the logic to handle this case and still
    compute the file offset correctly.
    
    Tested for x86_64.

diff --git a/libio/fileops.c b/libio/fileops.c
index e3967c1eb8ee6993..ac5ff9a4353d0201 100644
--- a/libio/fileops.c
+++ b/libio/fileops.c
@@ -858,17 +858,21 @@ libc_hidden_ver (_IO_new_file_sync, _IO_file_sync)
 int
 _IO_file_sync_mmap (FILE *fp)
 {
+  off64_t o = fp->_offset - (fp->_IO_read_end - fp->_IO_read_ptr);
   if (fp->_IO_read_ptr != fp->_IO_read_end)
     {
-      if (__lseek64 (fp->_fileno, fp->_IO_read_ptr - fp->_IO_buf_base,
-		     SEEK_SET)
-	  != fp->_IO_read_ptr - fp->_IO_buf_base)
+      if (_IO_in_backup (fp))
+	{
+	  _IO_switch_to_main_get_area (fp);
+	  o -= fp->_IO_read_end - fp->_IO_read_base;
+	}
+      if (__lseek64 (fp->_fileno, o, SEEK_SET) != o)
 	{
 	  fp->_flags |= _IO_ERR_SEEN;
 	  return EOF;
 	}
     }
-  fp->_offset = fp->_IO_read_ptr - fp->_IO_buf_base;
+  fp->_offset = o;
   fp->_IO_read_end = fp->_IO_read_ptr = fp->_IO_read_base;
   return 0;
 }
diff --git a/stdio-common/Makefile b/stdio-common/Makefile
index 9a5d324495946385..710120820ee4e337 100644
--- a/stdio-common/Makefile
+++ b/stdio-common/Makefile
@@ -211,6 +211,7 @@ tests := \
   tst-fdopen2 \
   tst-ferror \
   tst-fflush-all-input \
+  tst-fflush-mmap \
   tst-fgets \
   tst-fgets2 \
   tst-fileno \
diff --git a/stdio-common/tst-fflush-mmap.c b/stdio-common/tst-fflush-mmap.c
new file mode 100644
index 0000000000000000..3bddb909e072caf2
--- /dev/null
+++ b/stdio-common/tst-fflush-mmap.c
@@ -0,0 +1,50 @@
+/* Test fflush after ungetc on files using mmap (bug 32535).
+   Copyright (C) 2025 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <https://www.gnu.org/licenses/>.  */
+
+#include <stdio.h>
+
+#include <support/check.h>
+#include <support/temp_file.h>
+#include <support/xstdio.h>
+#include <support/xunistd.h>
+
+int
+do_test (void)
+{
+  char *filename = NULL;
+  int fd = create_temp_file ("tst-fflush-mmap", &filename);
+  TEST_VERIFY_EXIT (fd != -1);
+  xclose (fd);
+
+  /* Test fflush after ungetc (bug 32535).  */
+  FILE *fp = xfopen (filename, "w");
+  TEST_VERIFY (0 <= fputs ("test", fp));
+  xfclose (fp);
+
+  fp = xfopen (filename, "rm");
+  TEST_COMPARE (fgetc (fp), 't');
+  TEST_COMPARE (ungetc ('u', fp), 'u');
+  TEST_COMPARE (fflush (fp), 0);
+  TEST_COMPARE (fgetc (fp), 't');
+  TEST_COMPARE (fgetc (fp), 'e');
+  xfclose (fp);
+
+  return 0;
+}
+
+#include <support/test-driver.c>